Corrosion inhibition in aqueous ammonia-ammonium nitrate systems



May 17, 1960 A MARSH ET AL CORROSION INHIBTTION IN AQUEOUSAMMONIA-AMMONIUM NITRATE SYSTEMS Filed May 20, 1958 CORROSION m H20 NH3-NH4NO5 SYSTEM GELAT'INOUS r- CORROSION 1 pnooucrs l TE T SP OIMEN N O INHIBI TOR FIG. I

I N HIBI TOR INHIBITED SOLUTION NO OILY LAYER FIG.3

OILY LAYER 7471, L NH. no, H20 INHIBITOR INHIBITED SOLUTION UMJHIBITEDOILY LA YER FIG.5

OILY 1.10010 LAYER NO INHIBITOR OILY LIOUID LAYER FIG.2

OILY LIOUID INHIBITOR ONLY OILY LAYER 0 ON TAINS INHIBITOR FIG.4

/0ILY LAYER I NH IBI TOR INHIBITOR BOTH SOLUTION AND OILY LAYER OONTAININHIBITOR FIG. 6

INVENTORS GLENN A. MARSH EDWARD SOHASCHL A TTORNEY United States PatentCORROSION INHIBITION IN AQUEOUS AM- MONIA-AMMONIUM NITRATE SYSTEMSApplication May 20, 1958, Serial No. 736,616

17 Claims. (Cl. 206-84) This invention relates to new and usefulimprovements in methods for preventing corrosion of ferrous metals byaqueous ammonia-ammonium nitrate systems and is more particularlyconcerned with a disposition of corrosion inhibitors which inhibitscorrosion both below and above the liquid level.

Ammonia-ammonium nitrate systems containing water are utilized in thepreparation of some types of liquid fertilizers for commercial use. Suchsystems present a severe corrosion problem for the apparatus in whichthey are handled. Ferrous metal processing equipment as well as tankcars and other types of metallic shipping containers are subjected tovery rapid corrosion by the solutions far exceeding that encountered byferrous metal in contact with an ordinary aqueous environment. A specialcorrosion problem occurs where aferrous metal is in contact both withair or other oxidizing environment and the corrosive ammonia-ammoniumnitrate solution, as when a ferrous metal container is only partlyfilled with the solution and has an air space above the liquid level.The type of oxidation which occurs in this environment is unique andrequires a type of protection not needed in other corrosive environmentsand which is not suggested by previous investigators of corrosion. Whena ferrous metal is placed in contact with an aqueous ammonia-ammonium Initrate solution which contains no corrosion inhibitor, the rate ofcorrosion is very rapid. The corrosion products which develop on thesurface of the .ferrousmetal form a gel which adheres to the surface ofthe metal and grows rapidly in thickness as the corrosion of the metalsurface proceeds. Thus, when a thin strip of iron or steel is placed inan aqueous ammonia-ammonium nitrate solution very thick layers of oxidegels develop on each side of the strip and within a few days exceed thevolume of the metal strip. In fact, it has been noted in some cases thatwhen a metal strip is placed in a small jar containing an aqueousammonia-ammonium nitrate solution and allowed to corrode, the gelatinouscorrosion products may fill the entire'jar.

The problem of corrosion in aqueous ammonia-ammonium nitrate solutionsis only partially solved by 'the use of ordinary corrosion inhibitors.Inhibitors which are placed in the solution retard corrosion only in theliquid and have no effect on corrosion which occurs above theliquidlevel.

It is therefore one object of this invention to provide a means forinhibiting corrosion of ferrous metals by corrosive aqueous ammoniumnitrate-ammoniasolutions 'both in the air space above the liquid and onthe porrous metal container containing an aqueous ammonia- 2,93%,884Patented May 17, 1960 rosion inhibitor dissolved therein, and having aliquid layer floating on and substantially immiscible with the solution,in an amount sufficient. to cover the surface of the solution, andhaving dissolved therein a corrosion inhibitor substantially insolublein the solution, in an amount operable to inhibit corrosion of the wallsof the container above the liquid level.

A further featureof this invention is the provision of a ferrous metalcontainer containing an aqueous ammonia-ammonium nitrate solution havingdissolved therein as a corrosion inhibitor a compound containing asulfur-carbon-nitrogen linkage and having a mineral oil or other inertoleaginous liquid floating on the solution with a polar compounddissolved therein for inhibiting corrosion above the liquid level.

A still further feature of this invention is the provision of animproved method for inhibiting corrosion in aqueous ammonia-ammoniumnitrate solutions by dissolving in the solution a corrosion inhibitorand placing on the surface of the solution an inert oleaginous liquidhaving a corrosion inhibitor dissolved therein to inhibit corrosionabove the liquid level.

Other objects and features of this invention will become apparent fromtime to time throughout the specification and claims as hereinafterrelated.

In the accompanying drawings, to be-taken as part of the specification,there are illustrated diagrammatically the results of a series of testsdemonstrating this invention, in which drawings:

liquid level in an aqueous ammonia-ammonium nitrate ammonium nitratesolution provided with a suitable cor'- easily corrodible in thepresence of oxygen.

solution containing no inhibitor,

Fig. 2 is a diagrammatic view similar to Fig. 1 showing the extent ofcorrosion in a non-inhibited aqueous j ammonia-ammonium nitrate solutionhaving an oily liquid layer floating on the surface thereof,

Fig. 3 is a diagrammatic view showing the extent of corrosion of a testspecimen in an aqueous ammoniaarnmonium nitrate solution containing aninhibitor in said solution, but without the oily layer of Fig. 2,

Fig. 4 is a diagrammatic view similar to Fig. 3 showing the extent ofcorrosion of a test specimen in an aqueous ammonia-ammonium nitratesolution containing no, inhibitor but having an oily liquid floating onand covering the surface of the solution and having an inhibitordissolved therein,

Fig. 5 is a diagrammatic view similar to Fig. 3 showing the corrosion ofa test specimen in an aqueous ammonia-ammonium nitratesolutioncontaining a dissolved inhibitor and having an oily layer floating onand covering the surface thereof with no inhibitor in said layer, and

t Fig. 6 is a diagrammatic view showing a test specimen in an aqueousammonia-ammonium nitrate solution containing a dissolved'inhibitor andprovided with an oily layer floating on and covering the surface thereofwith an inhibitor dissolved in said layer.

We have found that it is possible to inhibit very substantially thecorrosion of ferrous metals by aqueous ammonia-ammonium nitratesolutions through the use of a single selected corrosion inhibitor wherethe metal has not been activated, i.e., has not been abraded or exposedto a strong acid or electrolytically coupled with a more active metal,to render the ferrous metal more Corrosion of ferrous metal in anunactivated state may be inhibited by a single selected corrosioninhibitor (e.g., 2-mercaptobenzothiazole) as long as the metal iscompletely immersed in the solution or the portion thereof above theliquid level is exposed to an oxygen-free atmosphere. It the ferrousmetal is activated, it may still be'inhibited nast est i i has againstcorrosion by a single inhibitor in the solution, if it is completelyimmersed in the solution and the solution is totally deaerated or theportion of the metal above the liquid level is exposed to an oxygen-freeatmosphere. metal is in an activated state, there are certaincombinations of inhibitors which may be dissolved inthe solution toinhibit corrosion, as longas the ferrous metal is totally immersed inthe solution or the portion thereof which is exposed is in contact withan oxygen-free atmosphere. While these conditions for inhibitingcorrosion of ferrous metals are of considerable theoretical interest, itis obvious that the conditions above enumerated are ones which aredifficult to attain in practice and thus are notapplicable to theinhibition of corrosion in aqueous ammonia-ammonium nitrate solutionsunder varying conditions.

The essence of our invention, therefore, resides in our discovery of ameans to inhibit corrosion in aqueous ammonia-ammonium nitrate solutionsregardless of the state of activation of-the ferrous metal, the degreeof aeration of the solution, or the presence of an oxygencontainingatmosphere in contact with a portion of the metal. We have found thatthe corrosion of ferrous metals by aqueous ammonia-ammonium nitratesolutions is turique. This corrosion takes place in the form of anoxidation at the surface of the metal, with the oxide forming a gelwhich adheres to the metalsurface. The rate of oxidation is rapidand-the gelatinous oxida' tion products build upto very substantialthicknesses before dropping from the metal. This form of corro-' sionand growth of'gelatinous corrosion products occurs both below and abovethe liquid level. test specimen in the form of a rectangular strip ispartially submerged in an aqueous ammonia-ammonium nitrate solutioncontaining no corrosion inhibitor, as in Fig. 1, the gelatinouscorrosionproducts which form may grow to athickness several times thatof the'test specimen. If one or more inhibitors is added to thesolution, as in Fig. 3, the formation of the gelatinous corrosionproducts is substantially completely inhibited and the corrosion of themetal surface is reduced to a very small fraction of that observed inthe absence of an inhibitor. This inhibition of corrosion, however,takes place only below the liquid level. The portion of the testspecimen protruding above the liquid'level continues to developgelatinous corrosion products thereon as shown in Fig. 3. If a layer ofan inert oily liquid is placed on the surface of the solution, with noinhibitor in the solution but a suitable inhibitor dissolved in the oilylayer, the growth of gelatinous corrosion pro-ducts above the liquidlayer is substantially completely inhibited while the gelatinouscorrosion products continue to grow on, the

portion of the test specimen extending into the solu-.

tion, as shown in Fig. 4. If an aqueous ammonia-ammonium nitratesolution is provided with an inhibitor and blanketed with an oily layercontaining an inhibitor dissolved therein, as in Fig. 6, a test specimenplaced in the solution and extending into the air space above will of anoily liquid layer without an inhibitor on an aqueous ammonia-ammoniumnitrate solution containing a dissolved inhibitor, as shown in Fig. 5,does not appreciably reduce the growth of gelatinous corrosion productsabove the liquid level. i

In carrying out this invention, the aqueous ammonia-.

ammonium nitrate solution which is to be stored in a ferrous metalcontainer is provided with a suitable corrosion inhibitor (preferably ina concentration of about If the solution is aerated and the ferrous If ametal Gill-0.2 wt. percent) which is soluble. therein. Any corrosioninhibitor may be used which is soluble in water and which substantiallymitigates corrosion by the solution. Compounds which are particularlyuseful in inhibiting corrosion by aqueous ammonia-ammonium nitratesolutions are mainly sulfur-containing compounds and preferablycompounds containing a sulfur to carbon to nitrogen linkage. Any of thevarious, sulfur compounds disclosed in U.S. Patent 2,238,651 to Frank G;Keenen may be used for inhibiting corrosion in the solution. Thesecompounds include oxides of sulfur, metal sulfides, sulfur chlorides,sulfamide, sulfurous acid and metal sulfites, metal hyposulfites, metalthiosulfates, metal salts of polythionic acids, persulfuric acid, andmetal persulfates, metal sulfates and bisulfates. Organic compounds ofsulfur which are particularly useful in inhibiting corrosion in aqueousammonia-ammonium nitrate solution include mercaptans, mercaptals,hydrocarbon sulfides and disulfides, sulfoxides, sulfonium compounds,sulfur derivatives ofhydroxy organic acids, sulfonic 'acids andderivatives thereof, thiosulfonates, thiocyanates, isothiocyanates,thiazoles, sulfur derivatives of carbonic acids, sulfur derivatives ofcarboxylic acids, and sulfur dye colors such as sulfogene compounds-andthe like. As pointed out above, the preferred inhibitors are ones whichinclude asulfur to carbon to nitrogen linkage and include specificallyZ-mercaptobenzothiazole, ammonium thiocyanate, thiourea, sodiumdimethyldithiocarbamate, trimethyl-' sulfonium nitrite, tetramethylammonium hydrosulfide, tetramethyl ammonium sulfide, and thioacetamide.These inhibitors may be used eithersingly in solution orin mixturesof-two or more or in" mixture with other corrosion inhibitors such aswater soluble metal nitrites, such aspotassiumnitrite; sodium:- nitrite,lithium nitrite, cesium nitrite, barium nitrite, and/ or inorganicoxidizing agentsfor ferrous metals such'as ammonium or alkali metalchromates'or chiorates.

' The oily layer which is;iloated"on the surface of the" aqueousammonia-ammonium nitrate solution is chosen strictly forits physicalproperties and chemicaljinertuess. The, layer consists of a liquid whichis lighter thanthe solution so that it may float on the solution andcover the entire surface thereof. The layer is of a liquid which issubstantially immiscible with thesolution and which is inert toward air,toward the solution, andtowardfthe' inhibitors which are used in thesolution and in the layer. The immiscible liquid layer ispreferably'a'hydrocarbon or other oily material and maybe, for example,liquid" propane or butane, naphtha such as rubber solvent; tex-t tilespirits, 'V.M. & P., Stoddard solvent, kerosene, fueloils of anyviscosity and of a petroleum origin, gas oils of a petroleum-derivednature,'neut ral lubricating stocks or anyoiher hydrocarbon oil; Oilyesters may be used, provided that they are inert toward the solution.Syn thetic lubricants which are lighter than the solution and immiscibletherewith may be used. Such synthetic lubri-' cants include the siliconeoils, such as dimethyl siliconepolymers, ester-type lubricants, such asdiisooctylazelate,- phosphate esters, such as tricresyl' phosphate,andlpolyoxyalkylene glycols, such as polypropylene glycols' of"sufliciently high molecular weight to be Water insoluble. Mineral oilssuch as a white oil are preferred. In the, oilylayer, there is dissolvedan inhibitor in a concentra-' tion of 0.1-5 wt. percent, withaconcentrationof 0.5 to

l.( wt. percent being preferred. The inhibitors which are andEngineering Chemistry, 40; 2338 2347'." Suitable inhibitors include C5-Cmonocarboxylic' acids, CFCIEQ. hydrocarbyl primary, secondary, and.tertiary' amines, C C alcohols, aliphatic and aromaticesters,.including monoesters of glycols and glycerol, metal "soapsgoff C-C3 aliphatic and aromatic facids, metal petroleum is a monoalkyl'-substituted trirnethylene diamine dioleate,

where the alkyl substituent is derived from tallow' acids and consistsmainly of C and C normal, saturated hydrocarbon chains, and Cmono-unsaturated hydrocarbon chains. Compound is prepared by directneutralization of DuomeenT (N-tallow-trimethylenediamine) followingstructural formula: v

with oleic acid. Io'nic ally, 'Duomeen TLDioleate has the[Alkyl-JF-CHa-GHz-CHa-IYEI 0111125000 H TH z Hereinafter throughout thespecification and claims, this compound (or mixture of compounds) willbe referred to V as: N-tallow-trimethylenediaminedioleate or DTD.

of the metaland collected in the bottom at the iarf low the. liquidlevel the metal had corroded at a rate of Other suitable oil-solubleammonium and amine salts of fatty acids include ammonium oleate,ammonium stear'ate,'ammonium palmitate, and trimethylamine oleate,triphenylamine oleate, ,stearylamine palmitate, oleylamine ricinoleate,stearylamine stearate, etc.

i The oily liquid layer and corrosion inhibitor therefor may be addedseparately, or with the inhibitor dissolved in .the oily liquid, andbefore, during, or after the addition of the water-soluble inhibitor tothe aqueous aminer ia-ammonium nitrate solution. Thus, any order andrate of addition of the individual'ingredients or groups of ingredientsmay be followed by, with or without agitation .ofthe solutionPreferably, thewater-soluble inhibitor is added to apreformed solutionof'amm-onia and ammonium nitrate in water, followed by the oily liquidcontaining acorrosioninhibitor dissolved therein.

4' A series of experiments was carriedout, using the procedure describedhereafter, forestablishing the scope of this invention and the necessityof using the combination of inhibitors in the manner above described. Aone pint glass jar was filled about half full of a solution consistingof ammonium nitrate, 60 wt. percent, am monia, 24 wt. percent, andWater, '16 wt. percent. The conrosivity of'this solution wastested on astrip of coldr'olled mild steel. A strip 'of cold-rolled mild steel 6.5"x .625" x .05" (l8 g auge was cleaned by rubbing with steel wool andactivated by being dipped in 15% HCl at 150 F. for 15 seconds. Thesolution in the jar was shaken until thoroughly aerated and cooled tore-; duce the vapor pressure as low as possible. The cleaned. andactivated steel strip was. placed in the jar with approximately half ofthe strip extending above the liquid level. The jar was then closed andsealed,wit h a lid enclosing an air spaceover the liquid. The solutionwas allowed to warm to room temperature and was maintainedat roomtemperature for-a period of two weeks. At the end of the two-week testperiod, the test specimen was observed for extent of corrosion and wasremoved f-romthe jar and cleaned and measured for thickness to determinethe extent of corrosion during the test period. The corrosion-rates inthis and other experiments are expressed in terms of mils per year orm.p.y. and other experiments the corrosion of the test specimen wasmeasured below and abovethe liquid level. In this experiment, the testspecimen developed a very heavy layer of gelatinous corrosion productsas illustrated in Fig. 1 of the drawing. In fact, at the end of the testthe gelatinous corrosion products had become so bulky as to have brokenpartially away from the surface In this When the test specimen wasremoved and cleaned and measured for extent of corrosion, it was foundthat be- 218 m.p.y., while above the liquid level the corrosion rate was'm.p.y.

EXAMPLE I A series of experiments was carried out following the testprocedure above-described, but using a corrosion inhibitor dissolved inthe solution. The test specimens were maintained in the corrosiveenvironment for a period of two weeks, were observed as to the type ofcorrosion which took place, and were measured for extent of corrosionbelow and above the liquid level as above-described. In one experiment,the aqueous ammonia-ammoniurn nitrate solution was inhibited by theaddition of 2 -mercaptobenzothiazole in a concentration of 0.12 wt.percent. The steel test specimen was almost completely free fromcorrosion products below the liquid level but developed a gelatinousgrowth of corrosion products above the liquid level as shown in Fig. 3of the drawing. The rate of corrosion below the liquid level was 4.8m.p.y. while the corrosion rate above the liquid level was 15 m.p.y. Asecond test sample was placed in an aqueous ammonia-ammonium nitratesolution containing 0.12 wt. percent ammonium thiocyanate as inhibitor-As in the preceding case, the corrosion was inhibited below the liquidlevel while growthv of corrosion products above the liquid level wassubstantial. The rate of corrosion of the test specimen below the liquidlevel wasO m.p.y. while the corrosion rate above the liquid level was112" m.p.y. In a third experiment, the aqueous ammoniaammonium nitratesolution was inhibited with 0.12 wt. percent of thiourea. Again, thecorrosion of the test speci-.' men was inhibited only below the liquidlevel, as in Fig. 3.. The corrosion rate below the liquid level wasmeasured and found to be 0 m.p.y. while the corrosion rate above. theliquid level was 139 m.p.y.

EXAMPLE II In another experiment, an aqueous ammonia-ammonium nitratesolution was prepared as in the preceding examples. This solution wasplaced in a jar and blanketed with a A" layer of a white mineral oil.Neither the solution nor the mineral oil contained any inhibitor. Ametal test specimen was cleaned and activated as described in theprevious examples and was sealed in the jar, exposed half to liquid andhalf to air, for a period of two weeks. As in the preceding example, thesolution was aerated and the space above the solution was filled withair. At the end of the test period, the test specimen was found to havedeveloped a heavy layer of gelatinous corrosion products as illustratedin Fig. 2. When the test specimen was removed from the jar and cleanedand measured, it was found that the corrosion rate above the liquidlevel was 255 m.p.y. and below the liquid level was 225 m.p.y.

I EXAMPLE III A series of experiments was carried out as in Example Iabove using 2-mercaptobenzothiaz0le, ammonium thiocyanate, andthiourea,-respectively, as corrosion inhibitors and each of thesolutions was blanketed with a /2".' layer of white mineral oilcontaining no dissolved corrosion inhibitor. The concentration of theinhibitors in the aqueous solution was 0.12 wt, percent as in Example I.In each of the three experiments, corrosion was substantially inhibitedbelow the liquid level while there' was a small amount of corrosion inthe form of gelatinous corrosion products on the portions of the testspecimens ex tending above the liquid level, as illustrated in Fig.5.The test specimen in the solution inhibited with 2-merc' aptobenzothiazole was corroded below' the liquid leyel at a rate of 5.4 m.p.y. andabove theJiquid level at a assesses.

rate of 10.8 m.p.y. The test specimen in the solution. 7

inhibited with ammonium thiocyanate was corroded below the liquid levelat a.-rate,of'5. 4 m.p.y. and above'the liquidrlevel at a rateof 11.4m.p.y. The test specimen in the solution inhibited with thiourea wascorrodedabove I EXAMPLE IV Another'experimentwas carried out duplicatingthe conditions of;,Example II except that the mineral oil layercontained 1%. by wt. of N-tallow-trimethylenediaminedioleate(hereinafter referred to as DTD as a corrosion inhibitor. The steel testspecimen was cleaned and activated as in the) previous examples andsealed in the glass jar half exposed'to liquid and half to air. At theend of the two-week test period, the test specimen had corroded in themanner shown in Fig. 4 of the drawings. The corrosion rate above theliquid level was only 3.6 m.p.y. while the corrosion rule in theuninhibited liquid solution below the liquid level was 30 m.p.y.

EXAMPLE V A series of three experiments was carried out using theinhibitors of Examples I and III and the inhibited oil layer of ExampleIV. In each experiment the steel specimen was cleaned and activated asin the preceding examples. The steel specimens were placed in therespective solutions half exposed to liquid and half to air and sealedfor a period of two weeks. In each experiment, the specimens weresubstantially completely free from corrosionproducts' and remainedshiny. In the jar containing ,Z-mercaptobenzothiazole in the solutionand DTD in theoil layer, the corrosion rate of the test specimen belowthe liquid level was 2.4 m.p.y. and above the liquid level was 3.6m.p.y. In the test jar in which the solution was inhibited with ammoniumthiocyana'te and the oil layer with DTD, the corrosion rate of the testspecimen below the liquid level was 3.6 m.p.y. and above the liquidlevelwas 6' m.p.y. In the test jar in which the solution was inhibited withthiourea and the oil layer with DTD, th corrosion rate below the liquidlevel was 3 m.p.y. and above the liquid level was 6 m.p.y.

The above experiments demonstrate'the fact that the corrosion of ferrousmetals, such as iron and steel, in aqueous ammonia-ammonium nitratesolutions takes place as illustrated in the drawings. When a metal stripis exposed to an aqueous ammonia-ammonium nitrate solution containing nocorrosion inhibitor, a heavy growth of gelatinous corrosion producttakes place as shown in Fig. 1. The addition of an oily layer withoutany inhibitor does not reduce the rate of corrosion in either the liquidor the air space above the liquid. The addition of corrosion inhibitorsin the aqueous ammona-ammonium nitrate solution inhibits corrosion belowthe liquid level but has no effect on corrosion of metal in the airspace above the liquid. The use of a non-inhibited oil layer on aninhibited solution does not inhibit completely the corrosion in the airspace above the liquid. The use or an inhibited oil layer on an aqueousammonia-annnonium nitrate solution containing no dissolved inhibitorinhibits corrosion in the air space above the liquid but does notcompletely inhibit corrosion in the liquid. It is only when a corrosioninhibitor is used in the solution and in the oily; layer floating on thesolution that corrosion is inhibited substantially completely both inthe liquid and in the air, space: above theliquid.

EXAMPLE vr In another series: of three experiments, the effect of acombinationof inhibitors in the aqueous ammonia-ammonium nitratesolution was evaluated, without an oil layer, with a non-inhibited oillayer, and with an oil layer inhibited with 1% by wt. of DTD. In eachexperiment. the aqueous ammonia-ammonium nitrate solution was inhibitedwith 0.06 wt.. percent Z-mercaptobenzoinhibitors in theoil layer-wasinvestigated. In one exthiazole and 0.06 wt. percent'sodium chromate. Inthe experiments without an oil layer and'with a non-inhibited oil layer,the corrosion below theliquid level. was inhibe ited but a gelatinousgrowth of. corrosion. products 0c currediabove the liquid level.Withoutanoil layer, the corrosion rate below the liquidlevel was1.2'm.p.y. and

above the liquid level. was 15 m.p.y. ,With a non-inhib-' ited oillayer, the corrosion rate below the liquid level was 0 and abovethegliquid levelwas 15.6 m.p.y. In the experiment withZ-mercaptobenzothiazole and sodium chromate in solutionand 1%" by wt. ofDTD in a /2" white oil layer floatingon thesolution, the growth ofgelatinous corrosion. products was inhibited both below and above theliquid level. The corrosion rate below the liquid level was 3' m.p.y.and above the liquid level was,5-.4 m.p.y.

. EXAMPLE VII In'another series of threeexperirnents the effectof'a':combination of three inhibitors; in the aqueous solution;

tassium chlorate. As in the other experiments, the solu.-' tion wasaeratedv and the test'zspecimens were exposed .to-

the aerated solution and to air in the space above the solution. As inExample VI, the experimentsusing no. oil layer and using an inhibitedoil layer were substan: tially identical in result. The growthofgelatinous corrosion productbelow the liquid. levelwas substantiallycompletely inhibited. while corrosion products. formed on:

the test specimens in the air space above the liquid. In.

the experiment using no. mineral oil layer. on the solution, the rate ofcorrosion. above the liquid level. was.

14.4 m.p.y; and below theliquid. level was 3 m.p.y. In the experimentusing a non-inhibited oil layer on the in;

t hibited solution, the corrosion rate above the liquid leveli was 13.8m.p.y. and below the liquid level was 3.6,m.p.y. In the thirdexperiment, in which a /2" mineral oil layer containing 1 wt. percent ofDTD wasfioated on the aqueous ammonia-ammonium nitrate solutioncontaining 2- mercaptobenzothiazole, sodium nitrite, and potassiumchlorate -as corrosion inhibitors, the test specimen wassubstantiallyfree of gelatinous. corrosion products both 'below and above the, liquidlevel. The, test specimen remained shiny and was only' slightlycorroded. Above the liquid level the rate of corrosion was 5.4 m.p.y.while below the liquid level the rate of corrosion was 1.21

m.p.y.v V

EXAMPLE .VIII

In another series of two experimentsthe eifect of other periment, 1 wt.percent of sodium ethylene diamine sulfonate was dissolved in mineraloil andapplied as a V2" layer over a non-inhibited -aqueousammonia-ammonium;

rosion products onthe test specimen below the liquid level wasapproximately the same as-in theexperiment" where no oil'layer was'used.The" corrosion rate'above the, liquid level was 48 m;p.y; and belowthe-liquid level was 193 m.p.y.

In the second? experiment, 1" wt. percent of glycerylmonooleate was'dissolved" in mineral oil and applied to a non-inhibited aqueousammonia-ammonium nitrate, solution as a /2 layer: The growthof'corrosion prod? ucts below the liquid level was' verygreat whilecorrosion was substantially"inhibited" (reduced by, 80%) above thefliquid iev e l. The corrosion rate abovejthe liquid level v inhibitorsof Example VIII in the oil layer was evaluated in connection with themixed inhibitors of Example VII in the aqueous solution. In oneexperiment the aqueous solution was provided with 0.04 wt. percent of2-mercaptobenzothiazole, .04 wt. percent of sodium nitrite, and 0.04 Wt.percent of potassiumchlorate. The solution was blanketed with a /2"layer of mineral oil containing 1% by wt.- of sodium ethylene diaminesul fonate. The test specimen was cleaned and activatedas in theprevious experiments. At the end "of the two-week test period, the testspecimen remained shiny and free from growth of gelatinous corrosionproducts. The corrosion rate both above. and belowthe liquid'level was3.6 m.p.y. r 1

In the second experiment an aqueous ammonia-ammonium nitrate solutionwas provided with 0.04 wt. percent of Z-mercaptobenzothiazole, 0.04 wt.percent ofsodium nitrite, and 0.04 wt. percent of potassium chlorate,and was blanketed with a /2" layer of white mineral oil containing 1 wt.percent of glyceryl monooleate. The test specimen was cleaned andactivated as in the previous examples. At the end of the two-week testperiod, the test specimen was substantially freeof corrosion productsand remained shiny. The corrosion rate of the test specimen above theliquid level was 6 m.p.y. and below the liquid level was 0.6 m.p.y.

EXAMPLE X In another experiment, a silicone oil (dimethylsiliconepolymer) was substituted for the mineral oil layer of Example VI. Theaqueous solution contained 0.06 wt. percent Z-mercaptobenzothiazole and0.06 wt. percent sodium chromate, while the silicone oil contained 1.0wt. percent of DTD. The corrosion in the liquid was substantiallycompletely inhibited while the corrosion above the liquid level wasreduced by about 30%.

EXAMPLE XI In two other experiments the conditions of Example V wereduplicated, substituting another synthetic lubricant for the mineral oillayer. In one experiment the solution contained 0.12%Z-mereaptobenzothiazole and the oily layer consisted ofdi-isooctylazelate containing 1% of DTD. Corrosion in the liquid wassubstantially completely inhibited. Corrosion above the liquid level wasreduced by about 75%. In the other experiment the solution contained0.12% Z-mercaptobehzothiazole a 10 Withthe use of each of the abovepairs of inhibitors corrosion above 'andbelow the liquid level .reducaiby more than 75%. v

From the above examples, it is seen .that protection against corrosionby aqueous ammonia-ammonium nitrate solutions, both below and above theliquid level may be accomplished, by incorporating in the aqueous,solution any, suitable corrosion inhibitor in an amount sufiicient toinhibit corrosion of the ferrous metal'con tainer below, the liquid,level and by blanketing the surface of the liquid with an oily layercontaining a suitable corrosion inhibitor dissolved therein in an amountsufficient to inhibit corrosion of the ferrous metal container in theair. space above the liquid level.

.While we have described this invention fully and completely as requiredby the patent statutes, including several preferred embodiments of theinvention, it is to be understood that within the scope of the appendedclaims, thisinvention may be. practicedotherwise than as specificallydescribed.

This application is a continuation-in-part of ourcopending applicationSerial No. 539,690, filed October 10,

Whatis. claimed is: 1 V

1. In combination, a ferrous metal container containing (A) an aqueousammonium nitrate solution containing dissolved ammonia and normallycorrosive to ferrous metals, said solution containing a corrosioninhibitor dissolved therein in an amount operable to inhibit corrosionof the walls of the container in contact with the solution, and (B) aliquid layer floating on and substantially immiscible with said solutionin an amount sufficient to cover the surface of said solution, saidliquid layer being of a material liquid at ambient temperatures,immiscible with, inert toward, and less dense than said solution, andsaid liquid layer having dissolved therein a corrosion inhibitorsubstantially insoluble in said solution in an amount operable toinhibit corrosion of the walls of the inhibitor in the aqueous solutionincludes at least one and the oily layer consisted of UCON LB 550 X(polypropylene glycol having a viscosity of 550 SUS at 100 F.)containing 1% of DTD. Corrosion in the liquid was substantiallycompletely inhibited. Corrosion above the liquid level was reduced byabout 60%.

EXAMPLE XII The procedure of Example V is followed substitutingcorrosion inhibitors in the solution and in the mineral oil layer asindicated in Table I. In each case the concentration of inhibitor in thesoluition is 0.12 wt. percent and in the mineral oil layer is 1%.

sulfur-containing compound.

4.. A combination as defined in claim 1 in which the inhibitor in theaqueous solution includes at least one compound having a sulfur tocarbon to nitrogen linkage.

5. A combination as defined in claim 1 in which the inhibitor in thefloating liquid layer is an oil-soluble polar compound. 7 I

6. A combination as defined in claim 1 in which the inhibitor in theaqueous solution is present in a concentration of about 0.01-0.2 wt.percent and the inhibitor in the floating layer is present in aconcentration of about 0.1-5.0 wt. percent.

7. A combination as defined in claim 6 in which the inhibitor in theaqueous solution includes at least one compound having a sulfur tocarbon to nitrogen linkage.

8. A combination as defined in claim 7 in which the inhibitor inthe'floating layer is a nitrogenous salt of a fatty acid of at least 6carbon atoms per molecule.

9. A combination as defined in claim 7 in which the inhibitor in theaqueous solution is Z-mercaptobenzothiazole.

10. A combination as defined in claim 7 in which the inhibitor in theaqueous solution is ammonium thiocyanate.

11. A combination as defined in claim 7 in which the inhibitor in theaqueous solution is thiourea.

12. In combination, 'a ferrous metal container containing (A) an aqueousammonium nitrate solution containing dissolved ammonia and normallycorrosive to ferrous metals, said solution containing at least 0.01 wt.percent of 2-mercaptobenzothiazole dissolved therein to inhibitcorrosionof the walls of the container in contact 7 withtli'e'solution, and (B) alayer of mineral oil'fioating on" and covering the surface of saidsolution, and said mineral oil having dissolvedtherein at least 0.1 Wt.percent of a polar compound to inhibit corrosion of the walls; of saidcontainer above the liquid level.

13'. In combination, a ferrous metalflcontainer containing ('A') anaqueous ammonium nitrate solution containing dissolved ammonia andnormally'j corrosive to ferrous metals, said solution containing atleast 0l01 wt. percent of Z-mercaptobenzothiazole dissolved therein toinhibit corrosion of the walls of the container in contact with thesolution, and (B) a layer of an oleaginous synthetic lubricantimmiscible with water, floating on and covering the. surface of saidsolution, and said' synthetic lubricant having dissolved therein atleast 0.1 wt. percent of' a polar compound to inhibit corrosion of thewalls of saidcontainer above the liquid level.

* 142 A combination as'defined'in claim 13 in which the oleaginoussynthetic lubricant is a stable organic ester.

15. A combination as defined in claim 13 in which .the

synthetic lubricantis a liquid oleaginous polyoxyalkylene "lycol; O

16. In combination, a ferrous metal container containing (A) an aqueousammonium nitrate solution containing dissolved ammonia.v andnormallycorrosive-to. ferrous corrosion oft the wallsof. the container incontact with the solution, and (B) a layer ofv mineral oilhavingidiS.

solvedth ereinianitrogenous salt of a fattyacid of at least 6carbonatoms per molecule in an amount suflT cient-to inhibit corrosionof thewalls of said; containerv ReferencesCited in the file of thispatent UNITED STATES PATENTS Hoover Aug, 21,1934- 1,993,773 ClarksonMar. 12,1935 2,215,092 Beekhtu's Sept. 17, 1940: 2,238,651 Keenen Apr.15,, 1941v 2,785,089 Lanteri Mar. 12, 195.7

12. IN COMBINATION, A FERROUS METAL CONTAINER CONTAINING (A) AN AQUEOUSAMMONIUM NITRATE SOLUTION CONTAINING DISSOLVED AMMONIA AND NORMALLYCORROSIVE TO FERROUS METALS, SAID SOLUTION CONTAINING AT LEAST 0.01 WT.PERCENT OF 2-MERCAPTOBENZOTHIAZOLE DISSOLVED THEREIN TO INHIBITCORROSION OF THE WALLS OF THE CONTAINER IN CONTACT WITH THE SOLUTION,AND (B) A LAYER OF MINERAL OIL FLOATING ON AND COVERING THE SURFACE OFSAID SOLUTION, AND SAID MINERAL OIL HAVING DISSOLVED THEREIN AT LEAST0.1 WT. PERCENT OF A POLAR COMPOUND TO INHIBIT CORROSION OF THE WALLS OFSAID CONTAINER ABOVE THE LIQUID LEVEL.